The present invention generally relates to measuring fluid perturbation. More particularly, the present invention relates to sensors, systems, and methods for measuring fluid height of linear waves and nonlinear waves.
Measuring fluid perturbation is important to scientists, researchers, and engineers. For example, wave measurements aid in understanding coastal dynamics. Indeed, wave measurements are important in determining the wave envelope, which gives information about incident wave amplitude and period, wave reflection, spatial variability of the wave field, and wave attenuation. All of the foregoing variables are used by scientists, researchers, and engineers to predict and explain coastal dynamics. Furthermore, liquid level measurements are important for hydrology and fluid processing in industrial settings.
Several prior art systems exist to measure fluid perturbation. For example, acoustic systems have been used to measure fluid perturbation. In an acoustic system, an acoustic sensor, such as an ultrasonic transducer, is located above a fluid. In operation, the acoustic sensor first transmits a sound wave toward the surface of the fluid. Then the acoustic sensor determines fluid perturbation by measuring the sound wave that reflects off the fluid surface.
Another example of a prior art system to measure fluid perturbation is a capacitance system. In a capacitance system, a capacitance probe, which is coated with a nonelectrical conducting material such as plastic, ceramic, or a polymer, is submerged in a fluid contained in a tank. The tank is grounded with a wire to earth ground. In operation, the capacitance probe is charged by passing a voltage from a control board. Fluid height is determined by measuring the time it takes to charge the capacitance probe because there is relationship between the time it takes to charge the capacitance probe and the fluid height in the tank.
Optical systems are still another example of a prior art system to measure fluid perturbation. In an optical system, cameras are installed around a fluid. In operation, fluid perturbation is determined from optical measurements.
The multilevel liquid sensing system disclosed in Wang, U.S. Pat. No. 4,382,382 is another example of a prior art system for measuring fluid perturbation. In the system of Wang, electronic signal detectors are coupled to typically 6 to 16 probe conductors. When a probe conductor is submerged in a liquid to be sensed, a digital output signal changes. The digital output signal is sent to an encoder and then presented to control circuitry of a device such as a home laundry and other household appliances.
The fluid level sensing system disclosed in Richards, U.S. Pat. No. 5,553,494 is another example of a prior art system for measuring fluid perturbation. In the system of Richards, typically up to 16 electrodes are installed in a pressure vessel to sense the resistivity of a fluid at different levels in the pressure vessel. The overall level measurement is accurate to about plus or minus 5 millimeters. While the typical electrode spacing is 50 millimeters, the system of Richards uses analog-to-digital converters to interpolate between electrodes to a resolution of about 1 millimeter. The system of Richards produces a digital output signal which may be used for control purposes of the pressure vessel.
The prior art systems to measure fluid perturbation have several disadvantages. Acoustic systems, for example, have a low sampling rate. In the acoustic system, one acoustic sensor usually only measures one fluid point. Accordingly, acoustic systems must use more than one acoustic sensor to measure more than one fluid point. When more than one acoustic sensor is used in an acoustic system, the acoustic system must coordinate the transmission of sound waves from each individual acoustic sensor towards the surface of the fluid to lessen the impact that sound wave interference has on measuring fluid perturbation. Considering the time it takes to coordinate the transmission of sound waves, acoustic systems have a sampling rate of about 10 Hz to 20 Hz.
Capacitance systems also have a low sampling rate. Between each fluid perturbation measurement the capacitance probe must be charged and discharged. Because the capacitance probe is electrically insulated it takes time to charge and discharge the capacitance probe. Furthermore, capacitance systems require frequent calibration, which is an inconvenience for users. Capacitance systems require frequent calibration because of drift in analog signals.
Optical systems that include cameras, for example, are difficult to use in field applications. While research laboratories have glass tanks where cameras may be installed, field applications may not have effective and convenient locations to install cameras. Accordingly, optical systems are not flexible systems.
Both the system of Wang and the system of Richards merely measure fluid perturbation for automation and control.
Importantly, spatial resolution and system cost are usually related to sampling rate in the prior art systems. For example, when the sampling rate is increased in a particular prior art system, either the cost of the system increases or the spatial resolution decreases.
To account for the trade-off between sampling rate, spatial resolution, and cost, the prior art systems for measuring fluid perturbation may generate data representing only the top-most fluid point or some equivalent or average of fluid points. Moreover, the prior art systems use few individual sensors. This measuring system is not accurate enough to resolve complex nonlinear fluid phenomena, such as breaking waves. For example, neither the system of Wang nor the system of Richards is accurate enough to resolve breaking waves. Inaccurate measurements of breaking waves leads to errors in understanding coastal dynamics including under-prediction of wave forces on structures as well as underestimating near bed stresses on the sediment bed, which leads to unforeseen coastal erosion issues.